With respect to SISO systems (the initials standing for “Single Input Single Output”) comprising a single transmit antenna and a single receive antenna, SIMO/MIMO systems advantageously make it possible, by means of spatial multiplexing, to increase the data transmission bitrate. It is recalled in this regard that the technique of “spatial multiplexing” consists in cutting the data stream to be transmitted into a certain number of sub-streams (thereby, advantageously, dividing the required passband accordingly), and then in simultaneously transmitting the various sub-streams, and finally in appropriately recombining the respective signals received on the receive antennas. The number of actually independent signals that it is thus possible to transmit by means of an MIMO system is equal to the rank of the transfer matrix H (matrix of dimension N·M, where N designates the number of receive antennas, and M the number of transmit antennas). The spectral efficiency of the system therefore depends on the number of independent signals, but also on the BER (the initials standing for “Bit Error Rate”) associated with each sub-stream—hence the benefit of furthermore envisaging a multi-antenna coding (called “space-time coding”) so as to reduce the BER.
It is recalled moreover that so-called OFDM modulation (the initials standing for “Orthogonal Frequency Division Multiplexing”) efficiently reduces the interference between data symbols. Moreover, OFDM is relatively fairly uncomplex to implement; in particular, OFDM signal modulation can be implemented in an efficient manner by means of an Inverse Fast Fourier Transform (IFFT), and the demodulation of an OFDM signal can be implemented in an efficient manner by means of a direct Fast Fourier Transform (FFT). However, OFDM modulation exhibits the drawback of being rather inefficient spectrally (because of the use of a “cyclic prefix”).
Another modulation, called “OFDM/OQAM”, has therefore been proposed which exhibits the advantage of being spectrally efficient (since it does not require any cyclic prefix, in contradistinction to OFDM), while circumventing (like OFDM) interference between symbols in a fairly uncomplex manner (the acronym OQAM is formed of the initials of the words “Offset Quadrature Amplitude Modulation”).
However, this quasi-absence of interference between symbols is obtained only in the case of SISO systems. Indeed, as demonstrated in the article by M. Payaró, A. Pascual-Iserte and M. Nájar entitled “Performance Comparison between FBMC and OFDM in MIMO Systems under Channel Uncertainty” (IEEE Wireless Conference 2010, Piscataway, N.J., USA, April 2010), the OFDM/OQAM modulation causes, in the case of SIMO/MIMO systems, interference between symbols, whose significance is all the larger the larger the errors (in practice inevitable) in the estimation of the channel.
In this same article, the authors propose a spatial multiplexing/spatial demultiplexing method intended to reduce such interference. More precisely, given a transmitter furnished with M transmit antennas and a receiver furnished with N receive antennas, the receiver obtains an estimate of the transfer matrix of the MIMO channel between the transmitter and the receiver for a given sub-carrier; thereafter, for this sub-carrier, a multiplexing vector (with M complex components) is determined which is proportional to an eigenvector of this estimate associated with the largest eigenvalue of this estimate; finally, the receiver determines a demultiplexing vector (with N complex components) for this sub-carrier by receiving pilot symbols dispatched by the transmitter, and by minimizing the mean square error over the symbols received after demultiplexing.
This method according to the article by Payaró et al. has the drawback of requiring very complex calculations, notably at the receiver level.